1. Field of the Invention
This invention relates to an active vibration isolation system (AVIS). Particularly, this invention relates to an AVIS having a direct pressure control on a pneumatic control system of the AVIS to isolate a mass from external disturbances, such as vibration. The AVIS is generally for use in a photolithography process.
2. Description of the Related Art
Photolithography is a process for manufacturing integrated circuits. In a photolithography process, light is transmitted through non-opaque portions of a pattern on a reticle, or photomask, through a projection exposure apparatus, and onto a wafer of specially-coated silicon or other semiconductor material. The uncovered portions of the coating, that are exposed to light, are cured. The uncured coating is then removed by an acid bath. The layer of uncovered silicon is altered to produce one layer of a multi-layered integrated circuit. Conventional systems use visible and ultraviolet light for this process. Recently, however, visible and ultraviolet light have been replaced with electron, X-ray, and laser beams, which permit smaller feature sizes in the patterns.
As the miniaturization of a circuit pattern progresses, the focus depth of the projection exposure apparatus becomes very small. More importantly, it is difficult to accurately align the overlay of circuit patterns of the multi-layered integrated circuit. As a result, a primary consideration for an overall design of the photolithography system includes building components of the system that achieve precision by maintaining small tolerances. Any vibration, distortion, or misalignment caused by external disturbances must be kept at minimum. These external disturbances affecting an individual part collectively alter the focusing properties of the photolithography system.
It has been proposed to provide an active vibration isolation system (AVIS). A conventional AVIS generally includes a pneumatic control system and an electronic control system. The pneumatic control system is capable of generating a large force, but has slow dynamics so that it cannot respond to high frequency disturbances or variations. Hence, the pneumatic control system supports a mass and is used to counteract low frequency internal disturbances and isolate the mass from high frequency ground vibration. The electronic control system compensates for any disturbance that the pneumatic control system does not sufficiently isolate, i.e., low frequency ground vibration and internal disturbances. In a lithography system, the mass may be a stage device and may be supported and isolated by a plurality of AVIS. In addition, more electronic control systems, operating in a horizontal direction, may be provided to isolate the mass from horizontal disturbances.
A conventional AVIS 100 is illustrated in FIGS. 1 and 2 having a pneumatic control system 102, shown in FIG. 1, and electronic control system 104, shown in FIG. 2. Pneumatic control system 102 includes a fluid-filled chamber 110 pressurized to support a mass 120. Mass 120 may represent an individual part, such as a stage device for holding the wafer, or may also represent the whole photolithography system. Pressurized chamber 110, generally known as a compliance chamber, acts like an air spring, while mass 120 acts like a piston compressing the fluid when the ground moves up and down or when mass 120 is disturbed or moves. To isolate mass 120 from vibration of the floor, the pressure inside compliance chamber 110 is preferably maintained at a level which counteracts the force of gravity on mass 120.
A damping system 130 is introduced into pneumatic control system 102 to minimize the movement of mass 120 as it rides on the air spring by allowing the fluid in compliance chamber 110 to pass through some type of resistance or restriction 132 into a damping chamber 134. The energy dissipated in restriction 132 provides damping of the natural behavior of mass 120 on the air spring. For optimal damping performance, damping chamber 134 may generally need to be as much as eight times the volume of compliance chamber 110.
In one embodiment, restriction 132 is a small hose connecting compliance chamber 110 to damping chamber 134. Alternatively, compliance chamber 110 and damping chamber 134 may be constructed of a single large chamber (not shown) with a wall dividing therebetween, the wall having a small hole acting as the restriction. Damping chamber 134 may be connected to a fluid supply 136 via a regulator 138, which controls the flow of fluid coming into or out of damping chamber 134. Controlling the flow adjusts the pressure level in damping chamber 134, which in turn stabilizes the pressure level in compliance chamber 110.
The electronic control system 104 shown in FIG. 2 includes an electronic actuator 150 connected to a motion sensor. The motion sensor generally includes a position sensor 152 and a velocity sensor 154. Position sensor 152 measures and provides a position error signal 156, while velocity sensor 154 measures and provides a velocity error signal 158 of the isolated mass 120. In the embodiment of FIG. 2, mass 120 is a base 124 over which a stage device 122 moves. Position and velocity error signals 156, 158, respectively, enter one or more feedback controllers. The embodiment of FIG. 2 shows a first feedback controller 160 receiving position error signal 156 and a second feedback controller 162 receiving velocity error signal 158. First and second feedback controllers 160, 162, respectively, in turn generate force signals 164, 166, which are used to isolate mass 120.
Electronic actuator 150 is also connected to a controller (not shown), such as a computer simulating a mathematical model, via a feedforward controller 172 to determine a calculated force signal 168, which is also used to isolate mass 120. A summing junction 174 calculates the difference between measured force signals 164, 166, and calculated force signal 168, and delivers the resulting signal 170 to electronic actuator 150. Electronic actuator 150 generates a force 176 corresponding to resulting signal 170, which is exerted on mass 120 to isolate it from disturbances that pneumatic control system 102 does not sufficiently isolate.
One problem with this conventional AVIS is that regulator 138 only indirectly controls compliance chamber 110 by controlling the fluid pressure in damping chamber 134. In the indirect control of compliance chamber 110, the control is influenced by a loss of the fluid between a measurement point (not shown) and compliance chamber 110. In addition, it is difficult to use a position information of mass 120 to control the pressure level in compliance chamber 110 since the relationship between the pressure level and the position information is not in exact proportion. Therefore, there is a need for an improved pneumatic control system whereby a direct control over the system is provided to better isolate the mass from external disturbances.
The advantages and purposes of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purposes of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and in accordance with the purposes of the invention, as embodied and broadly described herein, a first aspect of the present invention is a pneumatic control system to support a mass in a vibration isolation system. The pneumatic control system comprises a compliance chamber filled with a fluid to pneumatically support the mass, the fluid having a fluctuating pressure level caused by external disturbances, and a sensor device connected to the compliance chamber for determining a pressure information of the compliance chamber by directly measuring the pressure level in the compliance chamber. The pneumatic control system also comprises a controller connected to the sensor device for controlling the pressure level in the compliance chamber in response to the pressure information to minimize the effects of fluctuating pressure level in the compliance chamber.
A second aspect of the present invention is a pneumatic control device to support a mass in a vibration isolation system. The pneumatic control device comprises a compliance chamber filled with a fluid to pneumatically support the mass, and a pressure sensor connected to the compliance chamber for measuring a pressure level in the compliance chamber and for outputting a pressure information of the compliance chamber. The pneumatic control device also comprises a feedback control system connected to the pressure sensor for receiving the pressure information and generating a feedback control pressure signal, and a pressure control actuator connected to the compliance chamber and the feedback control system for controlling the pressure level in the compliance chamber in response to the feedback control pressure signal.
A third aspect of the present invention is a pneumatic control device to support a mass in a vibration isolation system. The pneumatic control device comprises a compliance chamber filled with a fluid pneumatically supporting the mass, the fluid having a fluctuating pressure level due to vibration of the mass, and a reference chamber filled with the fluid having a predetermined pressure level. The pneumatic control device also comprises a differential pressure sensor connecting the compliance chamber to the reference chamber, and measuring a differential pressure level between the compliance chamber and the reference chamber. Further, the differential pressure level is used to control the fluctuating pressure level in the compliance chamber so that the fluctuating pressure level is substantially eliminated.
A fourth aspect of the present invention is a vibration isolation system comprising a pneumatic control device and an electronic control device. The pneumatic control device provides a pneumatic support to a mass and isolates the mass from high frequency external disturbances. The pneumatic control device has a pressure sensor and a first feedback control system connected to the pressure sensor. The pressure sensor measures a pressure level in a compliance chamber and generates a pressure information of the compliance chamber. The first feedback control system adjusts the pressure level in the compliance chamber based on the pressure information so that the pressure level equals a desired value. The electronic control device isolates the mass from at least one of low frequency external disturbances and high frequency internal disturbances. The electronic control device has a motion sensor, an electronic actuator, and a second feedback control system. The motion sensor detects a motion information of the mass caused by the at least one of the external disturbances and internal disturbances. The second feedback control system is connected to the motion sensor and generates a force signal to control the electronic actuator. The electronic actuator is connected to the second feedback control system and exerts a force corresponding to the force signal onto the mass to counteract the at least one of the external disturbances and the internal disturbances.
A fifth aspect of the present invention is a vibration isolation system, comprising a pneumatic control device and an electronic control device. The pneumatic control device provides a pneumatic support to a mass and isolates the mass from high frequency external disturbances. The pneumatic control device has a differential pressure sensor connected to a compliance chamber and a reference chamber. The differential pressure sensor measures a differential pressure level between the compliance chamber and the reference chamber to control a pressure level in the compliance chamber. The electronic control device isolates the mass from at least one of low frequency external disturbances and high frequency internal disturbances. The electronic control device has a motion sensor, an electronic actuator, and a feedback control system. The motion sensor detects a motion information of the mass caused by the at least one of the external disturbances and internal disturbances. The feedback control system is connected to the motion sensor and generates a force signal to control the electronic actuator. The electronic actuator is connected to the feedback control system and exerts a force corresponding to the force signal onto the mass to counteract the at least one of the external disturbances and the internal disturbances.
A sixth aspect of the present invention is a method for pneumatically controlling vibration of a mass caused by external disturbances, the mass being pneumatically supported by a compliance chamber. The method comprising the steps of directly measuring a pressure level in the compliance chamber, and controlling the pressure level to minimize the effects of fluctuating pressure level in the compliance chamber.
A further aspect of the present invention is a lithography system comprising a pneumatic control system or a vibration isolation system as summarized in the above aspects of the invention.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. Additional advantages will be set forth in the description which follows, and in part will be understood from the description, or may be learned by practice of the invention. The advantages and purposes may be obtained by means of the combinations set forth in the attached claims.